There are numerous applications, such as scanner drives for infrared radar systems, where the drive motor must be capable of operating at extremely high speeds, typically, up in the range of 60,000 RPM. Although the mechanical load on the motor at running speed is usually modest, the motor must nevertheless be capable of producing considerable torque at lower speeds, in order to reach running speed in a reasonable period of time, i.e., less than 20 seconds. High speed motors are required for various uses which can be either constant speed or controlled variable speed applications.
Operating at extremely high speeds creates problems not only the need for low mechanical friction and precise balance, but also the need for low electromagnetic friction. At 60,000 RPM, for example, the switching rate of the magnetic flux is 1,000 hertz, or multiples thereof. If the magnetic material associated with the motor windings has substantial hysteresis or eddy current losses, there will be significant power losses at high speeds.
DC motors without rotating iron have been constructed so that they have negligible magnetic losses and smooth torque without cogging. However, such motors require brushes. Brushes are unacceptable in extremely high speed motors because of the mechanical friction of the sliding contact and because, at these high speeds, even the slightest eccentricity in the commutator surface results in brush bounce and unreliable contact.
Brushes can be eliminated in DC motors by using known brushless motor designs. Generally, a brushless DC motor includes rotating permanent magnets, stator windings, and a rotor position sensor to control solid state switching for commutating the windings. The windings are normally located in stator slots. In high speed motors the rotor should be as small as possible and this results in a relatively small slot opening on the inside diameter of the stator. As a result, such motors are difficult to construct and generally must be wound by hand.
Induction motors eliminate the need for brushes and are capable of operating at high speeds. These motors, however, suffer from similar stator winding problems as do the brushless DC motors. In addition, such induction motors have poor low speed torque and generally lack precise speed control capability.
An object of this invention is to provide a high speed brushless DC motor.
Another object is to provide an electric motor with very low electromagnetic losses.
Still another object is to provide a high speed motor which can readily be constructed using automatic winding machines.